1. Field of the Invention
This invention relates to radio direction finders used particularly for ships, vehicles and aircrafts.
2. Description of the Prior Art
The radio direction finders of the prior art include those in which a directional antenna itself, or a goniometer to which the output of the directional antenna is connected, is rotated, and the output thereof is combined with the output of a nondirectional antenna for the display of the direction of arrival of a radio wave, by means of a pointer reading on a direction-finder-bearing indicator, or by an image on a cathode-ray tube.
A method of rotating the antenna or goniometer by electric means is disclosed in, for instance, Japanese Patent Publication No. 42674/52, published on Oct. 26, 1977.
FIG. 1 in the aforementioned Japanese Patent Publication shows directional antennas 1 and 2, an antenna coupler 3, an electronic switch 4, a nondirectional antenna 5, a phase shifter 6 and a coupler 7. In this disclosed system, primary coils L.sub.01 to L.sub.08 and secondary coils L.sub.1 to L.sub.8 of the coupler 3 form a commonly termed electronic goniometer which is associated with the electronic switch 4 which, in turn, switches the outputs of these coils. With this mechanism, an output, for practical purposes, similar to that of a rotatable goniometer is obtained.
When antennas are installed in a locality surrounded by many nearby structures, as in ships, vehicles and aircrafts, or when Adcock antennas are used as directional antennas, a peculiar error determined by the construction and condition of installation of the antennas is produced due to the span of the antenna. Usually, the error is corrected mechanically or by inserting an attenuator in the directional antenna circuit.